1. Field of the Invention
This invention relates to a grease composition for use in fixed type constant velocity joints of vehicles.
The constant velocity joint is a part transmitting rotating motions at constant velocity even when operating angle variously changes at two intersecting shafts and is used in front-engine, front-wheel drive vehicles (FF vehicle) and 4-wheel drive vehicles (4WD vehicle). In these FF and 4WD vehicles, the transmission of driving force and the steering are carried out by front wheels, so that the use of the constant velocity joint is indispensable for smoothly transmitting the driving force even at a full steering state. In general, the fixed type constant velocity joint is used at a wheel side and is large in the operating angle, so that the lubricating conditions are very severe and the wearing is apt to be caused under a high contact pressure to bring about premature failure.
More particularly, the invention relates to a grease composition for constant velocity joint which can efficiently lubricate at such lubricating places to effectively prevent the wearing and has an excellent durability (flaking resistance).
2. Disclosure of the Related Art
Heretofore, lithium base extreme pressure grease containing sulfur-phosphorus series extreme pressure additive, lithium base extreme pressure grease containing molybdenum disulfide and the like have been used as a grease for the constant velocity joint used at a high contact pressure under wearable condition.
Since the use of FF vehicles as well as functional 4WD vehicles is rapidly increased from a viewpoint of weight reduction, secure of living space and the like, constant velocity joints (CVJ) are widely used in these vehicles. In FIG. 1 is shown a Zeppa type joint used as a fixed type constant velocity joint among these CVJs. When the joint transmits a rotating torque at a state of taking an operating angle in the Zeppa type joint, complicated rolling and sliding motions are created in the fitting of a ball 5 between a track groove 2 of an outer member 1 and a track groove 4 of an inner member 3. In the fixed type constant velocity joints, the lubricating conditions become more severer together with the high output power of engine, high speed of the vehicle and the miniaturization and weight reduction of CVJ. The lithium base extreme pressure grease used from the old time is insufficient in the durability and also is required to improve the heat resistance.